Catalytic Hydrogen Transfer of Ketones over Ni@SiO2 Yolk−Shell

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Catalytic Hydrogen Transfer of Ketones over Ni@SiO2 Yolk-Shell Nanocatalysts with Tiny Metal Cores Ji Chan Park,† Hyun Ju Lee,‡ Jae Young Kim,† Kang Hyun Park,*,‡ and Hyunjoon Song*,† Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon, 305-701, Korea, and Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National UniVersity, Busan, Korea ReceiVed: January 12, 2010; ReVised Manuscript ReceiVed: February 25, 2010

In the present work, we report the synthesis of Ni@SiO2 yolk-shell nanocatalysts that bear tiny nickel cores with an average diameter of 3 nm. The nickel nanoparticles were coated with silica through the microemulsion method, and the resulting Ni@SiO2 core-shell nanoparticles were partially etched by acid treatment. The calcination of the particles yielded yolk-shell nanocatalysts with a uniform structure. The catalysts were employed for heterogeneous hydrogen-transfer reactions of acetophenone. The optimized reaction condition used 0.03 mol % of the catalysts in the presence of 20 mol % of base at 423 K for 30 min. Under this condition, the conversion reached 90%, and the TOF was calculated to be 6000 h-1, which is at least 60 times higher than the previous results using nickel catalysts. The catalysts were highly stable and more than six times recyclable without any loss of catalytic activity. This rational yolk-shell design of the nanocatalysts can be employed for various organic reactions with the advantages of high reactivity and reusability. Introduction In synthetic organic chemistry, reducing carbonyl functionality to alcohols has become one of the most fundamental and useful transformations.1 Several procedures have been developed, including the use of metal hydrides, dissolving metals, catalytic hydrogenation, and transfer hydrogenation.2 In particular, hydrogen-transfer reactions are known to be environmentally benign and have easy to handle reagents, and the reaction rates and specificity can be precisely controlled by choosing hydrogen donors. Effective hydrogen-transfer catalysts developed thus far have mainly focused on homogeneous systems using noble metal complexes, such as Ru, Rh, and Ir.3,4 Heterogeneous reactions with first-row metals are also considered to be potential catalyst systems because of their advantages in catalyst recovery and recycling, as well as their affordable prices. Yus et al. have recently employed nickel nanoparticles for hydrogen transfer of carbonyl compounds to corresponding alcohols by using isopropanol as a hydrogen donor.5-7 However, such a heterogeneous process needs to be improved because the usage of the catalysts is still high (10-20 mol %), and the catalysts are not sufficiently stabilized in solution. To improve the catalyst stability, bifunctional structures that contain both metal nanoparticles and metal oxide supports have been used to prevent severe aggregation of the active catalysts. The interface between the metal particles and supports generates specific interactions in many cases and enhances the thermal stability of the catalysts during the reaction. However, for hightemperature reactions above 573 K, tiny nanoparticles start to melt from the surface and migrate on the support in order to agglomerate together and yield larger particles. Even at low temperatures, the particles readily aggregate after the cycle of solution phase reactions. Recently, Somorjai et al. designed new * To whom correspondence should be addressed. E-mail: chemistry@pusan. ac.kr (K. H. P.), [email protected] (H. R.). † Korea Advanced Institute of Science and Technology. ‡ Pusan National University.

core-shell-type nanocatalysts that have active platinum nanoparticle cores inside porous silica shells.8 The silica shells behaved as a blocking layer that isolated each nanoparticle from the neighboring ones but penetrated the molecular reagents through their mesopores. We have also demonstrated that the Au@SiO2 yolk-shell nanostructure can act as an excellent catalyst for p-nitrophenol reduction in solution phase.9 The reactants and products came in and went out through the silica layers, and all reactions occurred in the vacant space between the metal cores and silica hollow shells. The diffusion rate through the silica layers and reactivity on the metal surface were modulated by appropriate chemical treatments, resulting in significant enhancement of total reaction activities compared with those of unmodified structures.10 This nanoreactor framework was very stable at high temperatures due to the fact that the active particles were not physically approachable with each other unless the silica layers were degraded. The Pt@SiO2 core-shell nanoparticles were catalytically active up to 1023 K in air. The Ni@SiO2 yolk-shell nanoreactors were also stable at high temperatures and exhibited good activities for steam methane reforming reactions running at 973 K, which are comparable to the performances of state-of-the-art commercial reforming catalysts.11 However, the metal core diameters in the 10-100 nm range had a limitation in showing the optimal activity under the present reaction conditions, although such metal@silica yolk-shell nanoparticles demonstrated their usefulness as a model catalyst both in gas and in solution phase reactions. In heterogeneous catalytic reactions, numerous factors, such as active particle size, metal loading, support interactions, and additives,significantlyinfluencereactionactivityandselectivity.12,13 In particular, small particles are generally more active in catalytic reactions because the surface area-to-volume ratio and number of active sites (edges, kinks, and steps) on the surface largely increase by particle size reduction.14-16 Most heterogeneous catalysts used in the industry have active metal particles with a diameter of a few nanometers for the optimization of

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activity. Consequently, the active metal particles of bifunctional nanocatalysts should be reduced up to a couple of nanometers in diameter in order to approach real catalytic processes. Herein, Ni@SiO2 yolk-shell nanoparticles that bear tiny nickel cores (3 nm in diameter) covered with porous silica hollow shells by silica coating through microemulsion and partial etching of the nickel cores were synthesized. This yolk-shell nanocatalyst exhibited excellent activities for hydrogen-transfer reactions of various ketones and was reusable several times without any loss in conversion efficiency. The Ni@SiO2 yolk-shell catalyst with large nickel cores (30 nm in diameter) was also synthesized in order to study the core size effect on catalytic activity. We anticipate that these metal@silica yolk-shell nanocatalysts with tiny metal cores can be applied to various organic reactions, such as oxidation, reduction, and C-C coupling, by simply adjusting metal components. Experimental Section Reagents. Nickel(II) acetylacetonate (Ni(acac)2, 95%), oleylamine (70%), trioctylphosphine (TOP, 90%), tetramethyl orthosilicate (TMOS, 98%), octadecyltrimethoxysilane (C18TMS, 90%), igepal CO-630, poly(vinyl pyrrolidone) (PVP, Mw ) 55 000), and 1,5-pentanediol (PD, 96%) were purchased from Aldrich. Ammonium hydroxide (NH4OH, 28% in water), hydrochloric acid (HCl, 35% in water), and cyclohexane (99.5%) were purchased from Junsei. All chemicals were used as received without further purification. Preparation of Nickel Cores. A mixture of Ni(acac)2 (0.77 g, 3.0 mmol) and TOP (4.0 mL, 9.0 mmol) in oleylamine (20 mL) was slowly heated from room temperature to 493 K for 15 min under an inert atmosphere and was allowed to stir at 493 K for 45 min. After the reaction, the resulting black mixture was cooled at room temperature, and the particles were precipitated by adding ethanol (40 mL) and centrifuging at 10 000 rpm for 20 min. Finally, the nickel nanoparticles were dispersed in cyclohexane. Synthesis of Ni(3 nm)@SiO2 Yolk-Shell Nanocatalysts. Cyclohexane (25 mL) was added into a mixture of igepal CO630 (8.0 mL) and NH4OH aqueous solution (1.0 mL) and stirred for 10 min. The nickel particle dispersion in cyclohexane (25 mL, 60 mM with respect to the nickel precursor concentration) was added into the mixture and allowed to stir rapidly for 30 s. TMOS (1.0 mL) and C18TMS (1.0 mL) were added, and the mixture was stirred for 1 h at room temperature. The resulting Ni@SiO2 core-shell particles were precipitated by adding methanol (10 mL), washed with ethanol thoroughly, and redispersed in ethanol. The ethanol dispersion (30 mL) of the Ni@SiO2 core-shell particles (50 mM with respect to the nickel precursor concentration) was treated with hydrochloric acid (10 mL, 0.45 M) at room temperature for 30 min. The product was collected by centrifugation at 10 000 rpm for 30 min and washed with ethanol thoroughly. After drying at room temperature, the powders were placed in the ceramic boat in a glass tube oven, heated at a ramping rate of 4 K/min to 773 K, and calcined at 773 K for 2 h under a hydrogen flow of 200 cc/min. After calcination, the resulting powders were cooled to room temperature and immediately submerged into anhydrous ethanol (20 mL) under a hydrogen environment in order to prevent surface oxidation of the nickel cores. Synthesis of Ni(30 nm)@SiO2 Yolk-Shell Nanocatalysts. For the synthesis of nickel nanoparticles, Ni(acac)2 (2.1 g, 8.0 mmol) and PVP (11 g, 96 mmol) were dissolved in PD (100 mL). The reaction mixture was slowly heated from room

Park et al. temperature to 473 K for 20 min under a nitrogen atmosphere and was stirred at 473 K for 4 h. The resulting solution was heated to 543 K for 1 h. After cooling to room temperature, the product was precipitated by adding acetone (100 mL) with centrifugation and washed with ethanol thoroughly. For silica coating, the ethanol dispersion (50 mL) of the nickel particles (0.16 M with respect to the precursor concentration) was added to NH4OH aqueous solution (4.0 mL, 7.4 M) and the mixture was stirred for 30 s. TMOS (0.20 mL) and C18TMS (0.20 mL) were simultaneously added, and the resulting mixture was stirred for 1 h at room temperature. The product was washed with ethanol and separated by simple decanting with a magnetic force. The ethanol dispersion (30 mL) of the resulting Ni@SiO2 core-shell particles (0.27 M with respect to the precursor concentration) was treated with hydrochloric acid (11 mL, 1.0 M) at room temperature for 30 min. The product was collected by centrifugation at 6 000 rpm for 10 min and washed with ethanol thoroughly. After drying at room temperature, the resulting powders were calcined at 773 K under a hydrogen gas flow for 2 h. General Procedure for Hydrogen-Transfer Reactions. In the standard reaction condition, Ni@SiO2 yolk-shell nanocatalysts (2.0 mg, 0.030 mol % with respect to the substrate concentration), acetophenone (2.4 mL, 0.020 mol), isopropanol (10 mL), and NaOH (0.16 g, 4.0 mmol) were placed in a 25 mL stainless steel reactor. The reactor was tightly closed, and the mixture was stirred at 423 K for 30 min. The catalysts were separated from the clean supernatant by centrifugation. The reaction products were analyzed by 1H NMR. Characterization. The nanoparticles were characterized by Omega EM912 (120 kV, Korea Basic Science Institute) and Philips F20 Tecnai (200 kV, KAIST) transmission electron microscopes. The nickel particle samples were prepared by putting a few drops of the corresponding colloidal solutions on carbon-coated copper grids (Ted Pellar, Inc.), followed by drying in air. X-ray powder diffraction (XRD) patterns of nickel samples were recorded on a Rigaku D/MAX-RB (12 kW) diffractometer. The nickel loading amounts were measured by a MiniPal 2 energy-dispersive X-ray fluorescence (EDXRF) spectrometer. Proton NMR spectra were obtained using a Varian Mercury Plus (300 MHz). Chemical shift values were recorded as parts per million relative to tetramethylsilane as an internal standard and coupling constants in hertz. Results and Discussion Synthesis of Ni(3 nm)@SiO2 Yolk-Shell Nanoparticles. The synthesis of metal@silica yolk-shell catalysts basically followed a selective etching process of metal cores from metal@silica core-shell nanoparticles, as previously reported by our group.11 In the first step of the previous synthesis, the original nickel particles for the nickel cores were synthesized in a gram scale through a polyol process. However, the average particle size (37 nm) was relatively large for catalysis. The nickel core size could be reduced by partial etching with acids, but the resulting particles with diameters of 31 and 24 nm were still large compared with those of common metal nanocatalysts in a few nanometers scale. In the second step, the nickel particles were coated with silica via the Sto¨ber method.17 This method is good for large nanostructures, such as nanorods and wires, but is not applicable to smaller particles because the resulting silica layers on the particles are normally thick. Therefore, reducing the metal core size is not a simple modification of the previous synthetic process. Instead, it is reported that microemulsion in organic media can successfully generate metal@silica

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Figure 1. (a, b) TEM images, (c) SAED pattern, and (d) XRD spectrum of Ni nanoparticles. The bars represent (a) 20 and (b) 5 nm.

SCHEME 1: Synthesis of Ni@SiO2 Yolk-Shell Nanocatalysts with Tiny Nickel Cores

core-shell nanoparticles in a small size range.18,19 For this process, the metal particles should be passivated by organic nonionic surfactants in order to have stable dispersion during the coating process. We chose the nickel nanoparticle synthesis through thermal decomposition of nickel-oleylamine complexes because this method is via a simple one-pot reaction and can produce spherical nanoparticles with a narrow size distribution in organic media.20 The nickel precursor was mixed with trioctylphosphine and oleylamine, and the mixture was slowly heated and stirred at 493 K for a total of 1 h. After the reaction, the particles were separated by adding ethanol as a poor solvent and centrifugation. The resulting particles were dispersed well in organic solvents. Figure 1a shows a representative transmission electron microscopy (TEM) image of the spherical nickel particles with an average diameter of 5.1 ( 0.3 nm. The particles are polycrystalline with a couple of single crystalline domains (Figure 1b). A selected area electron diffraction (SAED) pattern (Figure 1c) shows a ring that corresponds to the (111) plane. The X-ray diffraction (XRD) pattern exhibits three broad signals that are

assignable to (111), (200), and (220), indicating face-centered cubic (fcc) nickel (JCPDS No. 04-0850). The mean particle size was estimated to be 2.2 nm by the Debye-Scherrer equation from the fwhm of the (111) peak, revealing the average singlecrystalline domain size of the polycrystalline nanoparticles. The next step is silica coating through the microemulsion method to fabricate Ni@SiO2 core-shell nanoparticles, as depicted in Scheme 1. The nickel particle dispersion in cyclohexane was mixed with igepal CO-630 in the presence of ammonia, which behaves as a catalyst during the decomposition of the silica precursor.21 A mixture of tetramethyl orthosilicate (TMOS) and octadecyltrimethoxysilane (C18TMS) was added to the reaction mixture in a volume ratio of 1:1 and was simultaneously stirred at room temperature for 1 h. C18TMS is a pore-generating reagent during silica polymerization. The TEM images in Figure 2a,b show a uniform coating of the silica layer on each nickel nanoparticle with an average thickness of 6.1 ( 0.5 nm. The particles are not aggregated but are isolated very well by the surrounding silica shells. The resulting core-shell particles are spherical and smooth on the surface. These

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Figure 2. TEM images of (a, b) Ni@SiO2 core-shell particles, (c, d) Ni@SiO2 yolk-shell particles, and (e, f) silica hollow shells. The bars represent (a, c, e) 50 and (b, d, f) 10 nm.

nanoparticles are dispersed well in ethanol, distinct from the original nickel nanoparticles in nonpolar solvents. The total average diameter of the core-shell particles is 17 ( 0.8 nm, which is quite smaller than that of core-shell particles (55 nm) using the Sto¨ber method. The nickel cores in the Ni@SiO2 core-shell particles were partially etched by hydrochloric acid treatment. Precise control of the etchant concentration could adjust the metal core size inside the silica shells. In the present experiment, careful optimization of the etchant concentration and volume was required in order to not dissolve all the particles because the core diameter was already ∼5 nm. When 0.45 M HCl solution was used, the average diameter of the nickel cores was reduced to 2.9 ( 0.3 nm. Figure 2c,d shows that the particles were still located inside the silica shell while bright regions were observed around the nickel cores, indicating vacant space between the core and shell in each particle. Increasing the HCl concentration to 1.1 M led to silica hollows with complete etching of the nickel cores (Figure 2e,f). The Ni@SiO2 core-shell and yolk-shell particles were treated at 773 K for 2 h in a hydrogen flow in order to burn out

all organic moieties, such as alkyl carbon chains of C18TMS and igepal CO-630, and to generate a clean metal surface and pores of the silica layers. The N2 sorption experiment at 77 K for the silica hollow shells exhibited a type IV adsorptiondesorption hysteresis, and the surface area and total pore volume were calculated to be 195 m2/g and 0.38 cm3/g, respectively. This means that the mesopores in the silica shells were successfully generated. Figure 3a,b shows that the silica framework maintained its spherical structure and all nickel cores are located inside without any agglomeration. In particular, the yolk-shell structure, including small particles and vacancy, is still stable at a high temperature of 773 K. Although the particle sizes remained unchanged in core- and yolk-shell particles, the peak intensities of the XRD spectra remarkably increased from those of the untreated samples. The particle sizes calculated from the fwhm of the (111) peaks at θ ) 44° by using the Debye-Scherrer equation were 4.2 and 3.5 nm for the coreand yolk-shell structures. In particular, the average particle size (3 nm) in the yolk-shell nanoparticles measured from the TEM images matches well with that from the XRD peak, indicating that the nickel cores are nearly single-crystalline. The particles

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Figure 3. TEM images of (a) Ni@SiO2 core-shell and (b) Ni@SiO2 yolk-shell particles after thermal treatment. (c) XRD spectra of Ni@SiO2 core-shell and yolk-shell particles before and after thermal treatment. All bars represent 50 nm.

TABLE 1: Hydrogen Transfer of Acetophenone to 1-Phenylethanol Catalyzed by Ni@SiO2 Yolk-Shell Nanocatalysts

entry

catalyst (mol %)

temp (K)

time (h)

base (mol %)

conv (%)a

1 2 3 4 5 6 7 8

0.05 0.05 0.05 0.05 0.05 0.05 0.03 0.03

423 373 373 353 373 373 423 373

0.5 0.5 1 1 1 2 0.5 0.5

20 20 20 20 10 10 20 20

93 58 92 61 68 87 90 83

a Determined by 1H NMR spectra. Yields are based on the amount of acetophenone used.

inside the silica shells melted and recrystallized to form better crystallinity during the heat treatment. The nickel loading contents were measured to be 27 wt % for the core-shell and 18 wt % for the yolk-shell particles by energy-dispersive X-ray fluorescence (EDXRF) spectroscopy, showing a 33% loss of metal components during the etching process. Hydrogen-Transfer Reactions by Ni@SiO2 Yolk-Shell Nanocatalysts: Effect on Particle Size. The Ni@SiO2 yolk-shell nanocatalysts were employed for catalytic hydrogen-transfer reactions. Acetophenone was used as a standard substrate in order to optimize the reaction conditions (Table 1). The reaction was carried out by using isopropanol as a hydrogen donor at 353-423 K in the presence of 10-20 mol % of NaOH.

According to previous studies, the catalytic hydrogen-transfer reactions proceed through two different pathways, that is, the direct transfer route by nontransition metals or hydridic route by transition metals.3 The reaction pathway by transition-metal catalysts involves a metal hydride formation.22 With respect to the substrate, the amounts of catalysts used for the reactions were 0.03-0.05 mol %. As a control experiment, the silica hollow shells without nickel cores did not proceed the reaction. The yield of the product, 1-phenylethanol, was highly dependent upon the reaction temperature. At 423 K, the yield reached to 93% within 30 min, whereas the reaction at 373 K showed 58% conversion in the same period (entries 1 and 2, Table 1). The reaction took 1 h to obtain a 92% yield at 373 K (entry 3, Table 1). The low-temperature reaction at 353 K gave a lower conversion efficiency of 61% after 1 h (entry 4, Table 1). The amount of base is one of the key factors of the reaction: the reduction of base concentration to 10 mol % diminished the yield to 68% after 1 h and 87% after 2 h reactions (entries 5 and 6, Table 1). Such a base effect was also observed in Ru-, Ir-, and Rh-catalyzed homogeneous reactions.23-25 The catalyst usage was also important in changing the reaction rates. For example, 0.03 mol % of the catalysts with respect to the substrate concentration was effective in running the reactions at 373-423 K. In particular, the reaction condition with 0.03 mol % of catalysts and 20 mol % of base at 423 K reached a decent 90% yield within 30 min without any byproducts (entry 7, Table 1). It was then chosen as a standard condition for further reactions. Turnover frequency (TOF) was calculated by the moles of 1-phenylethanol per moles of the nickel component per hour in this standard reaction condition. The TOF value, 6000 h-1, is the highest value among the previous results (100-500 h-1) that use heterogeneous nickel catalysts.6 With respect to the precursor concentration, most hydrogen-transfer

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Figure 4. (a, b) TEM images of Ni@SiO2 yolk-shell particles with a core diameter of 30 nm. The bars represent (a) 200 and (b) 20 nm.

reactions reported thus far generally used 10-20 mol % of the catalysts.26,27 This remarkable TOF is presumably due to the small size and surface cleanliness of the active catalysts. To check the particle size dependency of the reactions, Ni(30 nm)@SiO2 yolk-shell nanoparticles that have large nickel cores with a diameter of 30 nm were synthesized according to the previous study.11 The yolk-shell nanocatalysts were obtained through selective etching of Ni@SiO2 core-shell nanoparticles with an average core diameter of 37 nm. They were activated by thermal treatment at 773 K for 2 h under a continuous hydrogen flow. The TEM images in Figure 4a,b show that the nickel particles were clearly isolated by silica shells, while the average particle size and shell thickness were estimated to be 30 ( 3 and 6.9 ( 0.3 nm, respectively. The XRD pattern of Ni(30 nm)@SiO2 yolk-shell nanocatalysts shows sharp peaks for the fcc nickel reflections due to their large crystalline domains in the nickel cores. When these catalysts were employed in the hydrogentransfer reactions under the standard condition, the conversion yield and TOF were measured to be 81% and 5400 h-1, respectively, which is slightly less than those of the reactions that use the Ni(3 nm)@SiO2 yolk-shell nanocatalysts. Considering the relatively small surface area of the large nanoparticles, the reaction performances of the Ni(30 nm)@SiO2 yolk-shell catalysts are not bad, meaning that factors other than the active particle size dominate the heterogeneous hydrogentransfer reactions. Compared with the previous results that use small nickel nanoparticles with a diameter of ∼2 nm, the yolk-shell catalysts used here exhibited superior activities more than 60 times, mainly because the particle sintering during the reaction was completely blocked by the silica shells. The surface of the active nickel cores was also cleaned by burning all surfactants through high-temperature treatment, which rendered all surface atoms accessible to the substrate. In freestanding nanoparticles, the particle surface must be stabilized with the surfactants in order to prevent particle agglomeration in solution, but such surfactant passivation can block the active sites on the surface and significantly lower the reaction activity. Another advantage of the yolk-shell catalysts is being able to recycle the catalysts several times. For the repetitive reactions, the Ni(3 nm)@SiO2 yolk-shell catalysts were simply recovered by centrifugation after the reaction. The solvent and substrate were added to the catalysts, and the reaction proceeded again under the standard condition. As shown in Table 2, the catalysts were reused six times, but there was neither loss of conversion efficiency nor any byproducts. This is distinct from the freestanding nanoparticles, where the yield dropped to ∼60% after recycling the catalysts five to seven times. The silica shells in the yolk-shell nanocatalysts effectively stabilized the active

TABLE 2: Recycling Test of Ni@SiO2 Yolk-Shell Nanocatalystsa entry

catalyst

conv (%)b

1 2 3 4 5 6

0.03 mol % of original yolk-shell catalyst recovered from no. 1 recovered from no. 2 recovered from no. 3 recovered from no. 4 recovered from no. 5

90 92 89 89 90 90

a Reaction conditions: 0.03 mol % of Ni@SiO2 yolk-shells and 20 mol % of NaOH, isopropanol, 423 K, 30 min. b Determined by 1 H NMR. Yields are based on the amount of acetophenone used.

nickel nanoparticles during recycling. These high stability and reusability are expected to be an essential feature in the application of the catalysts for industrial processes that have large-scale production. Hydrogen-Transfer Reactions of Various Ketones: Effect on Substrates. The Ni@SiO2 yolk-shell nanocatalysts also showed high activities where various ketones were reduced under the standard reaction condition (Table 3). A series of aromatic carbonyl compounds were successfully reduced to the corresponding alcohols in the presence of isopropanol at 423 K, reaching 75-98% conversions within 30 min (entries 2-10, Table 3). Aliphatic rings, such as cyclohexanone, received hydrogen atoms from isopropanol in a 94% yield without any byproducts (entry 1, Table 3). Not only acetophenones but also other alkyl aryl ketones could be reduced with high conversions. Both steric and electronic factors determine the reaction properties of the hydrogen transfer. The propiophenone and 1-(naphthalen-2-yl)ethanone yielded 1-phenylpropan-1-ol and 1-(naphthalen-2-yl)ethanol, respectively, as sole products with good conversion yields (entries 2 and 4, Table 3). Diaryl ketones disfavored the reaction in low yields (entry 3, Table 3) because of their bulkiness. The steric effect of 1-(naphthalene-1yl)ethanol was more pronounced than that of 1-(naphthalene2-yl)ethanol, and thus, the reaction went on in a lower yield (entry 5, Table 3). The reduction of substituted acetophenones was affected by both the electronic property and the position of the substituent. Para-substituted acetophenones that contained electron-withdrawing groups showed a better conversion than those with electron-donating groups. The electron-withdrawing group (Cl) at the para position of the phenyl group enhanced the reduction reaction up to a 94% yield (entry 9, Table 3). On the other hand, electron donors at the para position showed yields of 75-80% (entries 6 and 8, Table 3). Interestingly, although the methyl group was similarly attached to the phenyl group, the ortho substituent showed a better yield (86%) than that of the para substituent (80%) (entry 7, Table 3).

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TABLE 3: Hydrogen Transfer of Various Ketones by Ni@SiO2 Yolk-Shell Nanocatalystsa

a Reaction conditions: substrate (0.03 mol), base (4.0 mmol), Ni@SiO2 yolk-shell (0.03 mol %), isopropanol (10 mL), 30 min, 423 K. b The conversion % was determined by 1H NMR spectra. Yields are based on the amount of the substrate used.

Conclusion Ni@SiO2 yolk-shell nanocatalysts that bear tiny nickel cores (core diameter, 3 nm; total diameter, 17 nm) were synthesized, which are meaningful for real catalytic processes. The nanocatalysts were synthesized by a microemulsion coating process of silica and subsequent partial etching of the nickel cores. These catalysts were employed for heterogeneous catalytic hydrogentransfer reactions of various ketones. Under a standard condition, the catalysts exhibited a remarkably high TOF value of 6000 h-1 in the reduction of acetophenone. This mainly resulted from the surface cleanliness, as well as the small size of the nickel

cores in the yolk-shell nanocatalysts. The catalysts were reusable several times without any loss of conversion yields. These supreme catalytic performances represent the importance of rational catalyst design by means of nanochemistry tools. This tiny yolk-shell nanostructure is expected to be a potential scaffold for heterogeneous catalysts in various organic reactions that can enhance the reaction activity and reusability. Acknowledgment. This work was supported by the Nano R&D Program (2007-02668) and by the Basic Science Research Program through the National Research Foundation of Korea

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(NRF) funded by the Ministry of Education, Science and Technology (MEST) (2009-0070926). References and Notes (1) (a) Trost, B. M., Fleming, I., Eds. ComprehensiVe Organic Synthesis; Pergamon: Oxford, U.K., 1991; Vol. 8, Chapters 1.1-1.8. (b) Hudlicky, M. Reductions in Organic Chemistry, 2nd ed.; The American Chemical Society: Washington, DC, 1996. (2) (a) Larock, R. C. ComprehensiVe Organic Transformations: A Guide to Functional Group Preparations, 2nd ed.; Wiley-VCH: New York, 1989. (b) Johnstone, R. A. W.; Wilby, A. H. Chem. ReV. 1985, 85, 129. (3) Ba¨ckvall, J.-E. J. Organomet. Chem. 2002, 652, 105. (4) Wu, X.; Liu, J.; Li, X.; Zanotti-Gerosa, A.; Hancock, F.; Vinci, D.; Ruan, J.; Xiao, J. Angew. Chem., Int. Ed. 2006, 45, 6718. (5) Alonso, F.; Calvino, J. J.; Osante, I.; Yus, M. Chem. Lett. 2005, 34, 1262. (6) Alonso, F.; Riente, P.; Yus, M. Tetrahedron Lett. 2008, 49, 1939. (7) Alonso, F.; Riente, P.; Yus, M. Tetrahedron 2008, 64, 1847. (8) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Nat. Mater. 2009, 8, 126. (9) Lee, J.; Park, J. C.; Song, H. AdV. Mater. 2008, 20, 1523. (10) Lee, J.; Park, J. C.; Bang, J. U. Chem. Mater. 2008, 20, 5839. (11) Park, J. C.; Bang, J. U.; Ko, C. H.; Song, H. J. Mater. Chem. 2010, 20, 1217.

Park et al. (12) Rolison, D. R. Science 2003, 299, 1698. (13) Bell, A. T. Science 2003, 299, 1688. (14) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (15) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Nørskov, J. K. J. Catal. 2004, 223, 232. (16) Haruta, M. Catal. Today 1997, 36, 153. (17) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (18) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (19) Osseo-Asare, K.; Arriagada, F. Colloids Surf. 1990, 50, 321. (20) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H.-J.; Park, J.-H.; Bae, C. J.; Park, J.-G.; Hyeon, T. AdV. Mater. 2005, 4, 429. (21) Chang, C.-L.; Fogler, H. S. Langmuir 1997, 13, 3295. (22) Trocha, J.; Henbest, H. B. Chem. Commun. 1967, 545. (23) Aranyos, A.; Csjernyik, G.; Szabo, K. J.; Ba¨ckvall, J. E. Chem. Commun. 1999, 351. (24) Kvintovics, P.; James, B. R.; Heil, B. Chem. Commun. 1986, 1810. (25) Uson, R.; Oro, L. A.; Sariego, R.; Esteruelas, M. A. J. Organomet. Chem. 1981, 214, 399. (26) Alonso, F.; Riente, P.; Yus, M. Synlett 2008, 9, 1289. (27) Kidwai, M.; Mishra, N. K.; Bansal, V.; Kumar, A.; Mozumdar, S. Catal. Commun. 2008, 9, 612.

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